FAQ 4.59 How can I know how many entries are in a hash?

This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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4.59: How can I know how many entries are in a hash?

(contributed by brian d foy)

This is very similar to "How do I process an entire hash?", also in
perlfaq4, but a bit simpler in the common cases.

You can use the "keys()" built-in function in scalar context to find out
have many entries you have in a hash:

my $key_count = keys %hash; # must be scalar context!

If you want to find out how many entries have a defined value, that's a
bit different. You have to check each value. A "grep" is handy:

my $defined_value_count = grep { defined } values %hash;

You can use that same structure to count the entries any way that you
like. If you want the count of the keys with vowels in them, you just
test for that instead:

my $vowel_count = grep { /[aeiou]/ } keys %hash;

The "grep" in scalar context returns the count. If you want the list of
matching items, just use it in list context instead:

my @defined_values = grep { defined } values %hash;

The "keys()" function also resets the iterator, which means that you may
see strange results if you use this between uses of other hash operators
such as "each()".

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